This invention relates to load and source pull testing of medium and high-power millimeter-wave (mm-wave) transistors and amplifiers.
Design of modern low noise or high-power mm-wave amplifiers and mixers, used in various communication systems, requires accurate knowledge of the active device's characteristics. In such circuits, it is insufficient for the transistors (device under test, DUT), which operate either as highly non-linear devices, close to power saturation, or at minimum noise conditions, to be described using linear (noise) or non-linear (power) numeric models.
A popular method for testing and characterizing such microwave components (transistors) in the non-linear region of operation is “load pull” or “source pull”. Load/Source pull (see ref. 1) is a measurement technique employing microwave impedance tuners and other microwave test equipment (FIG. 1), such as signal source 1, input and output impedance tuners 2, 4, (see ref. 7), power meter 5 and test fixture 3, which houses the DUT. In millimeter-wave frequencies (between 20 or 30 and 100 GHz) test fixtures are typically replaced by wafer-probes, allowing direct and low parasitic access to the semiconductor chips; other types of fixtures have too many parasitic components (packages, stray capacitors, bonding wires and lead inductors) that would mask strongly the real device. The tuners and test equipment are controlled by a computer 6 via digital cables 7, 8 and 9. The impedance tuners are instruments which allow manipulating the impedance presented to the Device Under Test (DUT) (see ref. 1) while testing and registering its behavior; this document refers hence to “impedance tuners”, in order to make a clear distinction to “tuned receivers (radios)”, popularly called elsewhere also “tuners” because of the included tuning circuits (see ref. 2).
When an automated electro-mechanical tuner is used in a setup as shown in FIG. 1, we speak of a passive load pull system. The typical configuration of the reflective probe inside the slabline is shown in FIG. 2: a reflective tuning element 21 also called “tuning” probe or slug, is inserted into the slotted transmission airline 24 and coupled capacitively with the center conductor 23 to an adjustable degree, depending from very weak 26 (when the probe is withdrawn) to very strong (when the probe is very close (within electric discharge—or Corona) to the center conductor 23; when the tuning probe moves vertically 26 between a “top position” and a “bottom position” and approaches the center conductor 23 and moves along the axis 25 of the slabline, it controls the amplitude and phase of the reflection factor seen at the slabline ports reaching areas covering parts or the totality of the Smith chart (the normalized reflection factor area). The relation between reflection factor GAMMA=|GAMMA|*exp(jΦ) and impedance Z is given by GAMMA=(Z−Zo)/(Z+Zo), where Z is the complex impedance Z=R+jX associated with GAMMA and Zo is the characteristic impedance of the system and slabline. A typical value used for Zo is 50 Ohms.
The tuning range (i.e. the capacity to reach the periphery of the Smith chart, or R=0 Ohm) of such a system is limited, essentially because of unavoidable insertion loss between the tuner and the DUT (FIGS. 1, 5 and 14), which means the passive tuner in the load pull system would, due to insertion loss, not reach the conjugate complex internal impedance and power-match many high power transistors, which have a small internal impedance (marked as “DUT” in FIG. 11 and “Conjugate complex DUT” in FIG. 5). To increase the tuning range of such a system one needs: (a) to reduce the insertion loss of the connection between DUT and tuner (FIG. 1) and (b) to insert, as close to the DUT as possible a pre-matching function (see ref. 2, FIG. 7 and FIG. 10). These two requirements translate into (a) a tuner airline (slabline) that extends beyond the tuner body and matches the angle of the wafer-probe (FIGS. 6, 13 and 15) allowing reducing the insertion loss and (b) a pre-matching module, which increases the tuning range at selected angles (FIG. 11).
Mechanical slide screw impedance tuners (FIG. 2 and FIG. 3) in the RF-microwave and millimeter-wave frequency range between 100 MHz and 110 GHz (see ref. 3, millimeter-wave frequencies are, typically, considered frequencies where the wave-length λ is best described in millimeters (mm); i.e. above 20 or 30 GHz (λ=300 mm/Freq(GHz), leading to λ(20 GHz)=15 mm and λ(30 GHz)=10 mm, and up to 110 GHz, λ=2.72 mm) are using the slide-screw concept and include a slabline 24, 30 with a test port 36 and an idle port 37, a center conductor 23, 31 (FIG. 3) and one or more mobile carriages 33 which carry a micrometric screw attached to a vertical axis 34 with manual handle or automatic (motor) control 35 which controls the vertical position of a reflective probe 32. The carriages are moved horizontally by additional manual handle or automatic (motor) control 37 and gear (ACME rod) 38. The signal enters one port and exits from the other. In load pull the test port is 36 where the signal enters, in source pull the test port is 39 where the signal exits. The typical configuration of the reflective probe 21 inside the slabline 24 is shown in FIG. 2. It requires a precise vertical axis 34 controlled by a micrometric screw 35. In the case of a manual pre-matching module however, the control is through a micrometric screw 170 in FIG. 17; pre-matching modules as in FIG. 17 cannot be integrated on the extended slabline (FIGS. 6 and 15) since the long vertical axis 34 with associated control screw 35 will conflict with the microscope (FIG. 19A); therefore pre-matching modules as in FIG. 17 cannot be mounted very close to the wafer-probe on the inclined portion of the slabline. The module will conflict mechanically with the microscope (area marked “CONFLICT AREA 63 in FIGS. 6 and 1502 in FIG. 15); therefore, the vertical axis, which is the main obstacle to the integration, must be eliminated.